I have had many issues with this same problem over the past few weeks and this solution was logical and it worked for me.
Xfinity - Aries TG
1. From browser (I used Firefox) go to 10.0.0.1
2. Enter (user name in my case) : admin
3. Enter password :(whatever yours is - default may be password; Password; or PASSWORD)
4: After logging on go to ;Troubleshooting; Reset/ Restore Gateway - Select RESTORE FACTORY SETTINGS
5. After restore you must enter WiFi settings as part of the process. USE SAME ID AND PASSWORD AS OLD SET UP
THI IS IMPORTANT AS IT SAVES A LOT OF REDOING EVERYTHING.
6. Log back in to your Router (10.0.0.1 from browser) mine reset to user: admin - and passord as : PASSWORD
7. It forced me to change password and I entered the old one.
Everything has been perfect since. Good luck to anyone who has this proble!
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